1. Field of the Invention
The subject invention relates to papermakers fabrics, in general, and to a double layer forming fabric having a tight bottom weft geometry, in particular.
2. Description of the Prior Art
In papermaking machines, paper stock, also called furnish or stuff, is fed onto the top surface or outer face of a traveling, endless, papermaking belt which serves as the papermaking surface of the machine. The bottom surface or inner face of the endless belt is supported on and driven by rolls associated with the machine. The papermaking belt, also known as Fourdrinier wire, forming medium, or forming fabric, is commonly configured from a length of woven fabric having its ends joined together in a seam to provide an endless belt. The fabric may also be constructed by employing an endless-weave process thereby eliminating the seam. Either fabric generally comprises a plurality of machine-direction yarns and a plurality of cross-machine-direction yarns which have been woven together on a suitable loom.
Initially, forming fabrics were woven-wire structures made from materials such as phosphor bronze, bronze, stainless steel, brass, or suitable combinations thereof. Recently in the papermaking field, it has been found that synthetic materials may be used, in whole or in part, to produce forming fabrics of superior quality. Today almost all forming fabrics are made from the following: polyester fibers, such as Dacron or Trevira; acrylic fibers, such as Orlon, Dynel and Acrilan; copolymers, such as Saran; or polyamides, such as Nylon. The warp and weft yarns of the forming fabrics may be of the same or different constituent materials and/or constructions and may be of monofilament or multifilament yarn of either circular or noncircular cross section.
In the prior art, numerous double-layer forming fabrics have been made by employing various 8- and 10-shaft weave designs. For example, see U.S. Pat. Nos. 4,182,381 and 4,359,069. These fabrics tend to have shortcomings related to the weave structure. A paper sheet coming off a papermakers fabric is wetter than would be expected from the apparent drainage rate designed into the fabric. Also fabric life tends to be lower than would be expected given the weave structure and the size of the yarns used.
Many of the prior-art forming fabrics employ cross-machine direction or weft floats on either the outer or the inner surface. As used herein, a float is a portion of a weft yarn that passes over (or under) two or more warp yarns, or it is a portion of a warp yarn that passes over (or under) two or more weft yarns before interweaving. For example, a weft or cross-machine direction yarn that passes over three warp or machine direction yarns before interweaving will be referred to herein as a three-float. These cross-machine direction floats when present on the inner surface of the fabric are quite loosely bound into fabric.
Two known fabrics employing a low-density double layer are made of an 8 shaft weave and a 10 shaft weave. The most notable difference between the two is that the cross-machine or weft yarns in the 8-shaft weave are stacked vertically, whereas the yarns in the 10-shaft weave are offset. It has been noticed in these types of fabrics that there are drainage problems which cause the resultant paper web to be too wet as it comes off the fabric. It has also been observed that such fabrics tend to exhibit a shorter fabric life than should be expected.
In studying these fabrics, especially the 8-shaft design, it has been noted that the machine-side weft yarns have a pronounced crimp which pulls the weft out of contact with the warp yarns except at the weft knuckles. As used herein, a knuckle is a one-float. The bottom weft is therefore bound quite loosely and exhibits what might be termed a loose bottom weft geometry. The resulting large gap between the bottom-weft floats and the warp yarns makes it difficult to get a good vacuum seal over the papermaking machine's suction boxes, rollers, etc. It is also apparent that the shape of the bottom-weft float leads to a high degree of wear at the center of the float and, hence, a poor life for the resultant fabric.
The loose bottom wefts occur on all fabrics which are woven from standard yarns since, during the heat-setting process when crimp interchange is taking place, the top-weft yarns have more crimps per unit length of yarn, thus forcing the bottom-weft yarns to take on a more pronounced crimp and move away from the warp yarns except at the crimp interchange.
It is toward solving or minimizing the prior-art problems that the present invention is directed